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    I recently started running into issues like this at work. There were two touch-screen laptops (Lenovo Yogas) that were part of a test harness and they would get phantom taps all over the screen sometimes. Display would go black occasionally and come back. There was a microcontroller that had run its firmware flawlessly that started having HardFaults (which is basically a segfault while handling a segfault) with a blown up stack and bizarre values in the fault registers. Finally I started realizing that this all seemed to start happening at the same time and suspected power. A regular $10 hardware store outlet tester didn’t indicate any issue, but with a multimeter the outlets had a measurable ~20V between the ground and neutral wires and the expected 120V between neutral and hot. This strongly implies that the building ground had become disconnected somewhere, probably at the breaker panel.

    This was all happening while we were getting ready to move anyway, so it’s going to remain a mystery forever. Taking the gear home (and then to the new shop) made all of the issues completely vanish.

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      I used to live in an apartment with no ground (and where the landlord apparently thought this was fine, despite it being a code violation). I wish I’d seen this story then so I could use this to tell her “look, do you want to be liable for damaged hardware”?

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        MacBook power bricks have two-prong connectors, so they’re not connected to ground at all; how would a grounding problem affect them? (Admittedly I’m not an expert on electricity.)

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          Even a strictly floating device, one that’s powered from a battery, for example, can end up accidentally grounded: you might, for example, touch its case while you’re resting on the ground. (Physical ground – not necessary the same as electrical ground potential – but in any case, while you’re at some different ground potential). So just because a device is powered by a two-prong connector doesn’t mean it’s never connected to any sort of ground.

          That’s not what this article is about though – from the sound of it, grounding the air conditioner fixed it, so the issue wasn’t related to the laptop being grounded, but to another device on the same circuit as it not being grounded. In that case, what could have been happening was that e.g. voltage spikes generated by the air conditioner’s power supply, or other transient overvoltages, weren’t filtered out, and other devices on the same circuit happily picked them up.

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            You can pop the two prong connector off and attach a grounded cable - there’s a big metal peg that the connector housing slides into for that.

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              I always do this – besides getting a grounded plug, it also means you don’t have the huge power brick taking up space on an outlet or power strip.

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                Do you actually know if it’s connected to anything on the inside?

                In the UK the earth pin is slightly longer and opens the safety shutters on the socket, so even floating power adapters need to have an earth pin. In this case the earth pin is usually plastic. Such devices (known as ‘Class II’ devices) obviously aren’t designed to earth anything.

                IIRC, many larger PSUs designed to take three pin power (e.g. ATX PSUs) are designed with Y-capacitors to earth. These are used to avoid discharging excessive EMI to the power lines as I understand it, presumably due to the large amounts of PWM modulation etc. involved in SMPS design. But it does mean you do actually have some electrical flow to the earth pin. If you have a lot of big switched-mode power supplies one wonders if this could add up to enough current to trip an RCD? Not sure.

                I’ve probably got something very wrong in the above paragraph since this isn’t my domain, hopefully someone else will show up and correct me…

                https://www.allaboutcircuits.com/technical-articles/safety-capacitor-class-x-and-class-y-capacitors/

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                  I’ve probably got something very wrong in the above paragraph since this isn’t my domain, hopefully someone else will show up and correct me…

                  You didn’t, and you raise a very valid point. It’s actually important for the ground pin to be (correctly) connected on both sides – both inside the device and at the installation end. Just because there’s a socket for the earth pin doesn’t mean there’s a cable that runs from the that to a ground plate.

                  Fun story: I worked in an office building that had a very poorly-executed installation. Wall sockets were not actually grounded (as in, the ground connector was not actually connected to an earth ground) and the circuit protection was not adequately designed, so plugging a JTAG connector into a mains-powered device from a mains-powered computer gave a very nasty spark, enough to, quite literally, blow a USB port, and leave an office floor in the dark for an hour by triggering the differential breaker in the main fuse box.

                  We were the only software development company in the building so nobody quite understood why these things happened. After the third occurrence or so, one of my colleagues exchanged a pretty good whisky for a spare key to the fuse box, so we could at least clean up the mess without having a maintenance engineer drive all the way there just to flip a breaker.

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              Oof. Grounding issues can make real weird things happen, but also only sometimes. I recall a guitar amp that had horrible noise and distortion when plugged into an ungrounded socket… but plugging it into a different ungrounded socket, it worked just fine. They’re IRL heisenbugs.